1. Field of the Invention
The invention relates generally to unmanned aircraft designs, and, more particularly, to aircraft designs that combine the features of helicopter and fixed wing aircraft.
2. Description of the Related Art
Some unmanned aircraft designs attempt to combine the vertical takeoff and landing (VTOL) and hover capabilities of a helicopter and the increased speed and range capabilities of fixed wing airplanes. Stop rotor “nose sitter” configurations, so named because the aircraft takes off and lands from a nose-down orientation, may offer good hover efficiency and aerodynamic design but can require complex mechanical systems. These designs can also suffer a significant loss in altitude during transition from helicopter to airplane mode, and involve uneven weight distributions, rendering the aircraft “top heavy” and unwieldy during takeoff and landing. Further, the counter-rotating fuselage and tail of some “nose sitter” designs are less practical than aircraft designs with a conventional fuselage orientation and tail rotor. “Tilt rotor” configurations with tiltable rotating propellers also involve mechanically complex systems and decreased hover efficiency due to higher disk loading. “Tail-sitter” designs, so named because the aircraft takes off and lands from a tail-down orientation, are associated with poor hover efficiency due to high disk loading and an awkward 90 degree attitude change between hover and forward flight modes.
The compound helicopter has a rotor system driven by an engine for takeoff, hovering, and landing and an additional propulsion system and supplemental wing independent of the rotor system. At higher speeds, the rotor system does not drive the aircraft and is substantially unloaded by the lift of the wing. Compound designs also have disadvantages: they are heavy due to additional systems and can suffer a significant download penalty when hovering due to the presence of the wing in the rotor downwash. The canard rotor wing configuration contemplates a rotor that stops in flight and acts as a fixed wing, but it too suffers drawbacks. The shape of its airfoil compromises between forward and reverse airflow directions, leading to reduced performance in both flight modes. Similarly, tilt duct designs, whose propellers are shrouded in ducts and rotate between flight modes, suffer from poor hover efficiency and high drag in forward flight mode. Thus, combining a helicopter's vertical takeoff and landing capability and efficient hover with a fixed wing aircraft's high speed and long range into one aircraft design while reducing or eliminating performance tradeoffs remains a significant aspect in aeronautical engineering.